I would take a step further back in time: Jack Scruby, Brigadier Peter Young, Charles Grant, Tony Bath, and (at the very beginning of the "modern era") H.G. Wells. There are, of course, many, many others.

This question does seem rather retro-spective. Those who shaped the hobby tend, by definition to be the founding fathers like Featherstone and his contemporaries. In many respects wargaming hasn't changed shape since then. However, there have been lots of game designers who have influenced the hobby since but who are possibly not so high profile. The type of games being played today are very different from those that were popular twenty years ago. Games tend to be smaller in terms of figure numbers, with rules that emphasise different things like the greater emphasis on command and control than previously. It seems a shame that we seem to look so far backwards when considering this question. With the exception of Priestly and Mustapha everyone else seems to be firmly lodged in a time capsule.

Phil Barker, long before DBA. WRG rules made Ancients gaming what it is today, for better or worse. Today Ancients Gaming is dominated by points based tournament rules, which is a legacy of WRG & DBX. My very first gaming armies were for WRG Tournament Play, and I had great fun. Little success. The upside of that is the availability of many armies for the gamer that nobody would bother to make if there was no market for them.

Georg Leopold von Reiswitz who wrote der Kriegspiel.His son Georg Heinrich Rudolf von Reiswitz who updated the work.Julius von Verdy du Vernois for speeding it up as Free Kriegspiel (the original game developer?)Helmuth von Moltke the Elder for "popularizing" the "game".

I would say that James Dunnigan and the designers at SPI had a greater impact on miniature wargaming than any four of the designers so far mentioned, as significant as they were. The reason being, Dunnigan basically laid out the wargame design concepts, methods and parameters that circumscribed the thinking of miniature wargame designers from the late 1970s onward. We are still working with [and struggling with] them today. Read his 1980 book The Complete Wargame Handbook and what he lays out about designing wargames, which has been republished several times with little editing. It is available on the web. PDF link